XUL vs. native toolkit has nothing to do with look and layout of the browser. All XUL is is an abstraction layer for the native toolkit. I'm right there with you if you want to talk about GUI responsiveness (which is something firefox could do well to improve on). Apps that use the native toolkit directly are going to be faster than ones that use an abstraction layer. But if you want to say that firefox is ugly because it's run through XUL is just not accurate.

Though abstracting several toolkits means that certain generalisations must be made, and inconsistencies will occur when compared to a native UI.

For instance, if Win32 supported something GTK didn't (or the other way round), support would need to be hacked in, and this might look somewhat ugly or not match themes or something. The same occurs with Swing and SWT on Java, or wxwindows.